Hetero-epitaxially grown compound semiconductor substrate and a method of growing the same

ABSTRACT

A method of growing a gallium arsenide single crystal layer on a silicon substrate comprises steps of growing a buffer layer of aluminum arsenide on the silicon substrate by atomic layer epitaxy, and growing the gallium arsenide single crystal layer on the buffer layer epitaxially.

BACKGROUND OF THE INVENTION

The present invention generally relates to fabrication of semiconductordevices and more particularly to an epitaxial growth of a compoundsemiconductor layer such as gallium arsenide on a silicon wafer.

Gallium arsenide (GaAs) is a typical compound semiconductor materialused for laser diodes and various fast speed semiconductor devices suchas metal-semiconductor field effect transistor (MESFET), high electronmobility transistor (HEMT), heterojunction bipolar transistor (HBT) andthe like because of its characteristic band structure and high electronmobility. Such a semiconductor device is constructed on a galliumarsenide wafer sliced from a gallium arsenide ingot grown as a singlecrystal or on a gallium arsenide substrate grown epitaxially on asurface of a silicon wafer. In the latter construction, one can avoidthe difficulty of handling heavy and brittle gallium arsenide waferduring the fabrication process of the device by using a light and strongsilicon wafer fabricated by a well established process for the base ofthe substrate. Further, one can easily obtain a large diameter wafer insuch a construction. As a result, one can handle the wafer easily andreduce the manufacturing cost of the device. Further, such a wafer issuited for fabrication of a so called optoelectronic integrated circuit(OEIC) devices wherein gallium arsenide laser diode and the like areassembled together with silicon transistors on a common semiconductorchip.

When growing gallium arsenide on silicon wafer epitaxially, however, oneencounters various difficulties. Such difficulties are caused mainly dueto large difference in the lattice constant and thermal expansionbetween silicon and gallium arsenide. For example, the lattice constantof silicon is smaller than that of gallium arsenide by about 4% and thethermal expansion coefficient of silicon is smaller than that of galliumarsenide by about 230%. From simple calculation based on the differencein the lattice constant, it is predicted that the gallium arsenidesubstrate constructed as such contains dislocations with a density inthe order of 10¹² /cm². Thus epitaxial growth of gallium arsenide layermade directly on silicon substrate is usually unsuccessful. Even ifsuccessful, such a layer involves significant defects such that theycannot be used as the substrate for a semiconductor device.

In order to eliminate these problems and obtain a gallium arsenidesubstrate layer having a quality satisfactory for a substrate ofsemiconductor device, it is proposed to interpose a buffer layer betweenthe silicon wafer and the gallium arsenide substrate so as to absorb anystress caused as a result of mismatch in the lattice constant andthermal expansion between the wafer and the substrate. In one example, asuper lattice layer is used for the buffer layer wherein a plurality ofcrystal layers each containing a few layers of atoms and having its ownlattice constant which is different from each other are stacked on thesurface of the silicon wafer before the deposition of the galliumarsenide substrate. By doing so, propagation of defects into the galliumarsenide substrate layer is prevented. Unfortunately, the formation ofsuch a super lattice structure requires an extremely precise control ofthe crystal growth which is difficult to achieve with reliability in thepresently available technique.

Alternatively, it is proposed to interpose a polycrystalline galliumarsenide buffer layer between the silicon substrate and the galliumarsenide layer to absorb the mismatching of the lattice constant andthermal expansion. In this approach, a thin gallium arsenidepolycrystalline buffer layer having a thickness of typically 10 nm isdeposited on the silicon substrate at a temperature of about 400°-450°C. prior to deposition of the single crystal gallium arsenide substratelayer. Then, the temperature is raised to about 600°-750° C. and thegallium arsenide substrate layer is deposited for a thickness of about afew microns. When the temperature is raised from the first temperatureto the second temperature, the polycrystalline gallium arsenide bufferlayer is recrystalized into single crystal and the gallium arsenidesubstrate layer deposited thereon grows while maintaining epitaxialrelation with the gallium arsenide buffer layer underneath.

In this technique, however, it is difficult to obtain a satisfactorilyflat surface for the single crystal gallium arsenide layer. This isbecause the polycrystalline gallium arsenide buffer layer takes anisland structure on the surface of the silicon wafer and the non-flatmorphology of the surface of the polycrystalline gallium arsenide bufferlayer is transferred to the gallium arsenide substrate layer providedthereon. In other words, the the surface of the gallium arsenidesubstrate layer becomes waved in correspondence to the island structureof the buffer layer. In spite of the use of reduced temperature at thetime of formation of the buffer layer so as to suppress the formation ofthe island structure by reducing the growth rate, the island structurecannot be eliminated satisfactorily. Further, such a waved surface ofthe gallium arsenide substrate cannot be eliminated even if thethickness of the gallium arsenide layer is increased to a few microns ormore.

Further, it is proposed to use other material such as silicon-germaniumsolid solution Si_(y) Ge_(l-y) for the buffer layer while changing thecomposition y continuously from the surface of the silicon substrate tothe bottom of the gallium arsenide substrate layer as is described inthe Japanese Laid-open Patent Application No. 62-87490. Alternatively,it is proposed to use a gallium arsenide based mixed crystal such asIn_(x) Ga_(l-x) As or Al_(x) Ga_(l-x) As with a composition x of about4.5×10⁻³ for the buffer layer (Japanese Laid-open Patent Application No.62-291909). In both of these alternatives, there is a problem in thesurface morphology as already described.

On the other hand, the applicants made a discovery during a series ofexperiments to deposit a group III-V compound such as aluminium arsenide(AlAs) on a gallium arsenide substrate by atomic layer epitaxy (ALE)that aluminium deposited on an arsenic plane of the gallium arsenidesubstrate rapidly covers the surface of the substrate with a surfacedensity corresponding to two or three molecular layers of the groupIII-V compound (U.S. Pat. application Ser. No. 172,671; Ozeki et al.,J.Vac.Sci.Tech.B5(4), Jul/Aug. 1987 pp. 1184-1186). Further, it wasfound that there is a saturation or self-limiting effect in thedeposition of aluminium arsenide. More specifically, there occurssubstantially no additional deposition of aluminium after it isdeposited on the surface of gallium arsenide for a surface densitycorresponding to two or three molecular layers of aluminium arsenide. Inthis study, however, it was not clear if such a self-limiting effectappears also when aluminium arsenide is deposited on the surface ofsilicon having diamond structure instead of the arsenic plane of galliumarsenide having zinc blende structure.

In the present invention, the applicants studied the hetero-epitaxialgrowth of group III-V compounds on silicon and discovered that epitaxialgrowth of a group III-V compound comprising at least one element havinga strong affinity with silicon can successfully eliminate the formationof the island structure when the compound is grown on silicon in a formof alternating atomic layers of the component elements.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful method of growing a compound semiconductor layer on awafer made of a single element and a semiconductor structuremanufactured by such a method, wherein the problems aforementioned areeliminated.

Another and more specific object of the present invention is to providea method of growing a substrate layer of a group III-V compound on asilicon wafer via a buffer layer of another group III-V compound foradjusting mismatch in the lattice between the substrate and the wafer,wherein formation of island structure in the buffer layer is effectivelysuppressed.

Another object of the present invention is to provide a method ofgrowing a substrate layer of a compound semiconductor on a siliconwafer, comprising steps of depositing a buffer layer including a firstelement having strong affinity with silicon and a second elementdifferent from the first element such that the first element and thesecond element are stacked on the wafer in a form of alternatingmonoatomic layers, and growing the substrate layer including a componentelement having a less stronger affinity with silicon on the bufferlayer. According to the present invention, the first element having thestrong affinity with silicon covers the surface of the silicon waferrapidly. Such a rapid coverage of the surface of the silicon wafer bythe first element is particularly facilitated as a result ofself-limiting effect when aluminium is chosen as the first element. As aresult of the rapid coverage of the surface of the wafer, the formationof the island structure in the buffer layer is effectively suppressedand the formation of the waved surface of the substrate layer incorrespondence to the island structure is effectively prevented.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read in conjuctionwith attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical view showing an apparatus used in the presentinvention for growing a gallium arsenide substrate on a silicon wafervia a buffer layer;

FIG. 2 is a graph showing a heat treatment employed in the presentinvention for growing the buffer layer and the gallium arsenidesubstrate layer on the silicon wafer;

FIG. 3 is a time chart showing a gas control sequence used for growingthe buffer layer and the gallium arsenide substrate layer on the siliconwafer;

FIG. 4 is a graph showing a result of Auger electron spectroscopyconducted for evaluating the degree of coverage of surface of thesilicon wafer by the buffer layer formed by the process of FIG. 3 incomparison with prior art corresponding structures;

FIG. 5 is a graph showing a result of Raman spectroscopy conducted forevaluating the quality of the surface of the gallium arsenide substratelayer obtained by the process of FIG. 3 in comparison with a prior artstructure; and

FIGS. 6(A)-(D) are drawings showing steps for growing the buffer layerand the gallium arsenide substrate layer on the silicon (100) plane byatomic layer epitaxy as shown in FIG. 3.

DETAILED DESCRIPTION

FIG. 1 shows an apparatus used in the present invention for growing agroup III-V compound substrate layer on a surface of a silicon wafer viaan intervening buffer layer using ALE based on a metal-organic chemicalvapor deposition (MOCVD) technique. In the embodiment described below,gallium arsenide is chosen as the group III-V compound and aluminiumarsenide is used as the material for the buffer layer, as this materialhas a lattice constant and thermal expansion close to those of galliumarsenide to be grown thereon. Further, both of the compounds are polarcompounds having ionic nature in the chemical bond. Because of thesereasons, it is known that there is an excellent conformity when thesetwo compound semiconductor materials are grown each other epitaxially.

Referring to the drawing, the apparatus has a chimney type reactionchamber 20 evacuated through a port 20a at its top, a susceptor 21 forheating a silicon wafer 22 held therein responsive to radio frequencyexcitation, a support pipe 23 for supporting the susceptor 21, anexcitation coil 25 for generating the radio frequency excitation, a gasmixer 26 for introducing a gas or a gas mixture into the reactionchamber 20, gas supply valves 27a-27d for introducing various source andpurge gases selectively into the reaction chamber 20, and a controller28 for controlling the supply of the gases through the valves 27a-27d.

At the beginning of the process, the silicon wafer 22 is baked at atemperature of about 1000° C. as illustrated in FIG. 2 by "PHASE I"under a reduced pressure so as to remove oxide film or any contaminationfrom its surface. The pressure of the reaction chamber 20 is maintainedat about 20 Torr and hydrogen in total of 2 SLM is flowed through thechamber 20 throughout the entire process. After about 20 minutes ofbaking, the temperature of the wafer 22 is reduced to about 500° C. in a"PHASE II" of FIG. 2 and an aluminium arsenide buffer layer is grown onthe surface of the wafer 22 by ALE.

FIG. 3 shows the sequence of gases introduced into the reaction chamber20 during the PHASE II process. Referring to FIG. 3, atrimethylaluminium gas ((CH₃)₃ Al) referred to hereinafter as TMA isintroduced first through the valve 27a with a flow rate of about 20 SCCMfor 7.5 seconds using a bubbler temperature of 22° C. The TMA thusintroduced is decomposed in the reaction chamber 20 and producesaluminium which covers the surface of the silicon wafer 22 as monoatomiclayer as will be described later. Next, the TMA gas or any residualcomponent species remaining after the deposition is purged from thereactor 20 by introducing hydrogen from the valve 27d for about threeseconds. Next, an arsine (AsH₃) gas diluted by hydrogen to 10%concentration is introduced into the reaction chamber 20 from the valve27b with a flow rate of about 480 SCCM for ten seconds. Arsine thusintroduced is decomposed in the reaction chamber 20 to form arsenic tobe deposited on the monoatomic aluminium layer on the surface of thesilicon wafer 22. After the introduction of arsine, the arsine gas orany residual component species remaining after the deposition is purgedfrom the reactor 20 by introducing hydrogen from the valve 27d. Withthis, one cycle of the gas sequence for the growth of the aluminiumarsenide buffer layer is completed. It is estimated that arsenicproduced as a result of decomposition of arsine is deposited on themonoatomic layer aluminium already deposited on the surface of the waferand causes rearrangement of aluminium. Thereby, two or three molecularlayers of aluminium arsenide is formed depending on the nature of thesurface of the silicon wafer.

After this, the gas sequence is repeated until a desired thickness ofthe aluminium arsenide layer is grown on the surface of the siliconwafer 22. The thickness of the aluminium arsenide layer is chosen to besufficient to relax the stress caused as a result of the mismatching inthe lattice constant and thermal expansion between the silicon wafer andthe gallium arsenide substrate. In one example, one hundred molecularlayers of aluminium arsenide are grown by repeating the gas sequence. Inthis case, the gas sequence is repeated for fifty times.

Next, the temperature of the wafer 22 is raised to about 600° C. and a"PHASE III" of the process shown in FIG. 2 is commenced. In this phase,a gallium arsenide substrate layer is grown on the aluminium arsenidebuffer layer on the silicon wafer by a suitable epitaxial growthtechnique such as the conventional MOCVD. In this process, it is notnecessary to introduce source and purge gases selectively but the sourcegas for gallium and arsenic may be introduced simultaneously. In oneexample, a gallium arsenide layer of about 2-3μm is grown by supplyingtrimethylgallium ((CH₃)₃ Ga) referred to hereinafter as TMG and thearsine gas with a flow rate of 2 SCCM and 40 SCCM, respectively forabout 2 hours.

FIG. 4 shows the result of Auger electron spectroscopy conducted on thesurface of the silicon wafer 22 in the various steps for covering thesurface by the aluminium arsenide buffer layer. In the drawing, theordinate represents a relative intensity of Auger electron emitted fromsilicon atoms in the surface of the wafer which is covered totally orpartially by the buffer layer with reference to a silicon wafer which isfree from coverage by the buffer layer. In other words, the relativeintensity of the Auger electron in the ordinate represents the degree ofcoverage of the surface of the silicon wafer by the buffer layer. Whenthe value of the ordinate is one, it means that the surface of thesilicon wafer is entirely exposed without coverage while when the valueis zero, it means that the surface of the silicon wafer is completelycovered by the buffer layer.

The abscissa of FIG. 4 represents the number of molecular layers ofaluminium arsenide deposited on the surface of the silicon wafer 22.Thus, the continuous line designated as "ALE AlAs" in FIG. 4 shows thatthe surface of the silicon wafer 22 is covered almost entirely with thealuminium arsenide buffer layer after it is deposited for about 20molecular layers. After the deposition of about 30 molecular layers ofaluminium arsenide, it can be seen that the surface of the silicon waferis totally covered by the buffer layer. This means that the surface ofthe silicon wafer is rapidly covered by the buffer layer withoutsubstantial formation of the island structure. The ideal coveragecalculated based on perfect layer-by-layer growth of the buffer layer isrepresented in FIG. 4 by a dotted line designated as "LAYER GROWTH". Itshould be noted that the coverage of the surface represented by the lineALE AlAs is quite close to the case of the ideal coverage represented bythe dotted line. In FIG. 4, the result obtained by a similar measurementfor the case in which a conventional gallium arsenide buffer layer isdeposited by a usual molecular beam epitaxy (MBE GaAs) is also presentedby a broken line for comparison. It can be seen that, in this case, thecoverage of the surface of the silicon wafer is still incomplete afterthe deposition of more than one hundred molecular layers. From thisresult, it is quite clear that there is a substantial formation of theisland structure or clustering of gallium and arsenic at the surface ofthe wafer.

In FIG. 4, the result of the process described above is further comparedwith the case in which a gallium arsenide buffer layer is grown on thesurface of the silicon substrate by the ALE (a continuous linedesignated as "ALE GaAs"). Even in comparison with this case, it isclear that the surface of the wafer is covered more rapidly whenaluminium arsenide is used for the buffer layer.

FIG. 5 shows a result of evaluation of the surface of the galliumarsenide substrate grown on the aluminium arsenide buffer layer incomparison with a gallium arsenide substrate grown on a gallium arsenidebuffer layer which in turn is grown on the surface of the silicon waferby ALE. The evaluation is made by irradiating an argon-ion laser beam onthe surface of the gallium arsenide substrate layer and observing aRaman scattering. In FIG. 5, the strong peak designated by "LO"corresponds to the Raman scattering attributed to the (100) plane ofgallium arsenide while the more diffused peak indicated as "TO"corresponds to the presence of other planes of gallium arsenide and/ordefects. It can be seen that only the (100) plane is observed in thecase of the gallium arsenide substrate grown on the aluminium arsenidebuffer layer indicating that the crystalline quality of the galliumarsenide substrate is sufficient, while distortion of crystal plane isobserved when the gallium substrate is grown on the gallium arsenidebuffer layer grown on the silicon substrate.

Next, the growth of the buffer layer and the substrate layer on thesilicon wafer will be described with reference to schematical crystalstructure diagrams of FIGS. 6(A)-(D).

Referring to FIG. 6(A), aluminium atoms formed as a result of pyrolysisof TMA in the phase II of FIG. 2 settle on the (100) plane of a siliconwafer I in a form of a monoatomic layer m as a result of its strongaffinity to silicon. In the following description, the term affinity isused as a qualitative measure representing the degree of chemical bondor magnitude of heat of formation of a compound formed when twodifferent elements are combined each other. Although the sites occupiedby the aluminium atoms is still hypothetical, aluminium atoms aredeposited with a surface density corresponding to two molecular layersof aluminium arsenide uniformly over the entire surface of the wafer Iwithout causing clustering. Qualitatively speaking, this phenomenonmeans that aluminium is more stable when it is combined with siliconthan it is clustered on the surface of the silicon wafer because of itsstrong affinity to silicon. Further, it was observed that there appearsa self-limiting effect similarly to the case of deposition of aluminiumon gallium arsenide as is reported previously by the applicants (U.S.Pat. application Ser. No. 172,671 by the present applicants), whenaluminium is supplied beyond the surface density corresponding to thetwo molecular layers of aluminium arsenide, although such aself-limiting effect of aluminium on silicon having diamond structure isfirst discovered in the study which forms the basis of the presentinvention. Further, it was found that the density of aluminium issaturated at a value corresponding to three layers of aluminium arsenidemolecules when it is deposited on the (110) plane of silicon similarlyto the case of the ALE on the gallium arsenide (110) plane.

Next, arsenic atoms formed as a result of the pyrolysis of arsine isdeposited on the aluminium monoatomic layer m. When the arsenic atomsreach the aluminium monoatomic layer m, the aluminium atoms arerearranged and there is formed the two molecular layers of aluminiumarsenide as shown in FIG. 6(B) by a layer II. As the initialdistribution of aluminium is uniform throughout the entire surface ofthe wafer I, there is no formation of the island structure in the layerII even if arsenic is deposited thereon. By repeating the supply of TMAand arsine with intervening purging by hydrogen as already described,the layer II is grown to a desired thickness and forms the buffer layer.In this layer II, the aluminium atom and the arsenic atoms are stackedalternately. After the formation of the buffer layer II, a galliumarsenide layer III is formed by an ordinary MOCVD as already describedor by molecular beam epitaxy (MBE) as shown in FIG. 6(C). As the galliumarsenide has a lattice constant and thermal expansion which are almostidentical to those of aluminium arsenide, there is no difficulty in thedeposition of gallium arsenide on aluminium arsenide thus covering thesurface of the silicon wafer. The overall structure of the galliumarsenide substrate thus obtained comprises the silicon wafer I, thealuminium arsenide buffer layer II and the gallium arsenide substratelayer III as shown in FIG. 6(D). As already described, the buffer layerII absorbs the stress caused as a result of mismatching in the latticeconstant and thermal expansion between the wafer I and the substratelayer III. As the aluminium arsenide has a similar lattice constant andthermal expansion to those of the gallium arsenide, the gallium arsenidesubstrate layer III thus grown is almost free from defects and can beused for a substrate of the compound semiconductor device withoutproblem.

Further, it was found that the growth of the buffer layer II may be madeby depositing arsenic first on the silicon wafer and then depositingaluminium. In this case, the sequence of TMA and arsine is simplyreversed from those of FIG. 3 without changing the duration or otherconditions for the ALE. In this case, too, the formation of the islandstructure in the buffer layer is effectively suppressed and the obtainedgallium arsenide substrate shows a flat surface. Although the mechanismof suppressing of the island structure in this case is not certain, onemay suppose that arsenic in the first monoatomic layer on the siliconwafer distributes uniformly over the silicon wafer, and then aluminiumis distributed uniformly over the arsenic monoatomic layer.

Further, the group V elements to be deposited on silicon as thecomponent element of the buffer layer II is not limited to arsenic butnitrogen (N) and phosphorus (P) both having stronger affinity to siliconthan gallium or arsenic constituting the substrate layer may be used aswell. When such a group V elements are deposited, they are distributeduniformly over the surface of the silicon wafer by rapidly combiningwith silicon and the island growth of the buffer layer is suppressed. Itshould be noted that the order of deposition of aluminium and such groupV elements in the buffer layer V during the ALE may be reversedsimilarly to the case of the ALE growth of the aluminium arsenide bufferlayer. When using nitrogen or phosphorus as the group V element, ammonia(NH₃) or phosphine (PH₃) may be used as the source gas for theseelements.

When the compound substrate layer to be grown on the silicon wafer isindium phosphide (InP) instead of gallium arsenide, it was found thatcompounds comprising elements such as aluminium, gallium and nitrogenhaving stronger affinity to silicon than indium or phosphorusconstituting the substrate layer is suitable for the buffer layer II.

Further, the ALE of the buffer layer is not limited to those describedwhich are based on MOCVD but molecular beam epitaxy may be used as longas the supply of the element can be made alternately. Furthermore, thegrowth of the substrate layer is not limited to MOCVD but conventionalmolecular beam epitaxy may be used as well.

Further, the present invention is not limited to these embodiments butvarious variations and modifications may be made without departing fromthe scope of the invention.

What is claimed is:
 1. A method of growing a gallium arsenide singlecrystal layer on a silicon substrate, comprising the steps of:growing abuffer layer of aluminum arsenide on the silicon substrate epitaxially;and growing the gallium arsenide single crystal layer on the bufferlayer epitaxially, said growth of the buffer layer being made by anatomic layer epitaxy process.
 2. A method as claimed in claim 1 in whichsaid step of growing the buffer layer comprises alternated deposition ofaluminum and arsenic wherein aluminum is first deposited on the siliconsubstrate.
 3. A method as claimed in claim 1 in which said step ofgrowing the buffer layer comprises alternated deposition of aluminum andarsenic wherein arsenic is first deposited on the silicon substrate. 4.A method of growing a layer of a compound semiconductor material on asingle component substrate of a first element, said method comprisingthe steps of:growing a buffer layer of another compound semiconductormaterial consisting essentially of second and third elements that aredifferent from said first element, each of said second and thirdelements forming a compound having a first heat of formation whenreacted with said first element, on a surface of said semiconductorsubstrate epitaxially such that the second and third elements arestacked in a form of alternating monoatomic layers, said growth of thebuffer layer being made by an atomic layer epitaxy process; and growingthe layer of the first mentioned compound semiconductor materialepitaxially on said buffer layer, said first mentioned compoundsemiconductor material comprising fourth and fifth elements each ofwhich forms another compound having a second heat of formation which issubstantially smaller than the first heat of formation when reacted withsaid first element.